The anti-idiotypic regulatory network comprised of anti-idiotypic (anti-id) T cells and antibodies is critical in the regulation of autoreactive T cells and B cells, and can be stimulated by immunization with inactivated autoreactive T cells (T cell vaccination). T cell vaccination with irradiated autologous myelin basic protein (MBP)-reactive T cells that are implicated in the pathogenesis of multiple sclerosis (MS) results in depletion of MBP-reactive T cells and correlates with clinical improvement in patients with MS. It has been shown that T cell vaccination induces anti-id T cell and B cell responses in recognition of the variable and hypervariable sequences of the T cell receptor (TCR) expressed by the immunizing MBP-reactive T cells in MS patients. This project is proposed to define the complete distribution pattern of the idiotypic epitopes and to identify the dominant idiotypic epitopes responsible for eliciting anti-id immune responses in T cell vaccination. In particular, the study is designed to evaluate whether TCR (hyper)variable regions most frequently used among MBP-reactive T cells in MS patients contain dominant idiotypic epitopes for anti-idiotypic B cells and T cells and whether TCR peptides incorporating the identified dominant idiotypic epitopes are effective in inhibiting MBP-reactive T cells by up-regulating anti-idiotypic immune responses. The project has three specific aims. Aim 1: Recombinant full-length TCR incorporating the most frequently utilized (hyper)variable regions among MBP-reactive T cells will be made to generate and select anti-idiotypic B cell lines and T cell lines from immunized MS patients. Aim 2: The resulting anti-id antibodies and T cell lines recognizing the full-length TCR will be examined to map the complete idiotypic epitopes and determine whether the (hyper)variable sequences contain the dominant idiotypic epitopes using overlapping TCR peptides. The antibodies and T cell lines will be further characterized for the reactivity to the whole immunizing T cells and the regulatory properties on the immunizing MBP-reactive T cells. Aim 3: The selected TCR peptides will be evaluated in vitro for the ability to stimulate anti-idiotypic B cells and T cells in relationship to their inhibitory effect on MBP-reactive T cells derived from MS patients. The results will help to determine the value of the TCR peptides in therapeutic regulation of MBP-reactive T cells in a subset of MS patients where the T cells express the common TCR sequence features. The long-term goal of the study is to define the mechanism of T cell vaccination and develop a practical peptide-based vaccination approach.